The bonding together of various surfaces by adhesion plays an important part in the formation of assemblies of materials. One concern here is that bonds are often only very difficult to undo again. This is often associated with the dissolution or destruction of the bonding layer.
Another possible way to bond surfaces together is via their dry adhesivity. Dry adhesivity herein is to be understood as meaning the formation of adhesion forces between the surfaces without adhesion-promoting materials, such as adhesive materials. Bonds of this type are also notable for being removable without residues.
Bonded systems of this type are frequently reliant on the surface evincing some structuring. However, a distinction must be made here between systems requiring a specific counter-structure, for example hooks and eyelets, and systems capable of developing adhesion forces with any desired surfaces.
Bonded systems of this type are also known for example in nature, as in the case of gecko or insect legs for example. It is believed that the bonding forces in systems of this type are based on van der Waals forces. The surface structuring in these systems leads to a very much larger area of contact and hence also to a very much higher level of adhesion forces being developed on contact.
In dry adhesivity, the magnitude of adhesion between two surfaces depends on the area available for adhesion. Two planar surfaces accordingly adhere to each other to distinctly better effect than one planar surface and one rough or nonplanar surface. The general rule is that the greater the area available for adhesion, the better the adherence between two surfaces.
However, this area is generally unalterable, so the adhesion force of a surface is generally not alterable.
However, materials are known from the prior art which are able to adopt different shapes as a function of external influences. Familiar materials of this type are, in particular, shape memory alloys (SMAs).
Shape memory alloys can change their crystal structure without changing their composition, and this change in crystal structure can be triggered thermally or mechanically. The change is usually triggered thermally, i.e., the shape memory alloy changes its structure on reaching a certain temperature.
Conventionally, the high temperature phase of a shape memory alloy is known as the austenite phase and the low temperature phase is known as the martensite phase. The basis for the special properties of shape memory alloys is that the ability of the two phases to transform into one another is reversible.
Two different possibilities are distinguished in this context. When an austenite article of specified shape is cooled down to below the transformation temperature, it transforms into a martensite article of the same shape. This article, then, is subsequently converted into a second shape without exceeding the critical strain of martensite (usually about 5-7%, depending on the alloy used). When this article is heated to above the transformation temperature, it transforms back into austenite and in the process readopts the original first shape. This process may also be repeated more than once. However, once again cooling the austenite article to below the transformation temperature without renewed deformation will not transform it into the second shape. This second shape is only obtainable by renewed deformation of the article. This effect is accordingly also called a “one-way shape memory effect”.
Different behavior is attainable when the shape memory material is deformed plastically or treated thermomechanically. Either will create in the material a microstructure causing there to be a memory not only for the shape of the austenite article but also for the shape of the martensite article. In consequence, cooling and transforming from austenite into martensite will likewise create a shape change to the structures at the surface. This process is reversible, so changing between these two shapes is possible by heating and cooling. This effect is also called a “two-way shape memory effect”.
It must be borne in mind here that the transformation of martensite into austenite need not take place at the same temperature as the transformation of austenite into martensite. Typically there will be some distinct hysteresis. In addition, the transformation takes place over a temperature range. The transformation temperatures are known as the martensite start temperature (Ms) and martensite finish temperature (Mf) and also as austenite start temperature (As) and austenite finish temperature (Af), where the capital letters designate the particular transformation product. That is, if austenite is cooled, Ms represents the temperature at which it will begin to transform into martensite. FIG. 1 shows a corresponding diagram. The relationship between these temperatures is therefore typically Mf<MS<AS<Af. The temperatures can vary if the shape memory alloy is under stress. This can raise or lower the transformation temperatures.
One particular version of this reversible effect is shown by DE 2010 034 954 A1, which shows the reversible formation of elevations on a surface for information storage.
It is also known to use shape memory alloys for tack-adhered bonds (e.g., U.S. Pat. No. 6,773,535). However, there the deformation of a component part in shape memory alloy is used to weaken the tack-adhesive bond.
The production of surface structures in the micrometer region that are switchable is comparatively recent. DE 10 2010 034 954 A1 describes the production of reversibly switchable depressions or elevations. These are used to store information on the surfaces that is to become visible on heating.
Using shape memory alloys to achieve switchable adhesion of surfaces is not known.
The problem addressed by the present invention is that of providing an article having switchable adhesion.